And yet, the twins stopped by Friday's episode of The Ellen DeGeneres Show. It marked the fashion designers' first time on TV since they appeared on the same daytime program in September 2010.
The 27-year-old style icons were there to promote their new fragrances, Elizabeth and James Nirvana White and Elizabeth and James Nirvana Black. While they were there, host Ellen DeGeneres seized the opportunity to quiz the Olsen twins about what sets them apart in a game of "Mary-Kate or Ashley?"
Though they became famous before they could talk, people still confuse them. "For us I think it's more like, 'Are you Mary or Kate?' or 'Are you Mary-Kate and Ashley?" Ashley explained. "I'm one of 'em!"
Mary-Kate and Ashley then agreed to play DeGeneres' game. The rules were simple: the comedienne would ask the twins a question, and they would flip a paddle to show which sister fit the question best.
Who is the messiest? Who is funniest? Who is best singer? Who got kissed first?
Watch to find out!